Terephthalic Acid.—Dissolve 5 grams/-toluic acid in dilute
caustic soda solution and boil with reflux condenser, adding 12
grams of permanganate in 250 c.c. water gradually from a tap
funnel inserted through the top of the condenser. When the red
colour of the permanganate persists after continued boiling the
solution is treated with sulphur dioxide (see p. 166), which dissolves
the manganese dioxide and precipitates the terephthalic acid as
a white amorphous powder. The latter is filtered, washed, and
dried. It sublimes without melting at 300'' and is insoluble in
water and alcohol. The yield is nearly theoretical.
CII3.C0H4.COONa+NaOH + 2KMnO4==
NaOOC.CGH4.COONa + 2KOH + MnO2 + 2H2O.
Diazoamiiiobenzene, C0H5N :N.NH.CBH5.
Gricss, Annalcn, 1866, 137, 58 ; Staedel, Bauer, Ber., 1886, 19,
20 grms. aniline.
6 „ cone, sulphuric acid.
600 „ water.
7*4 „ sodium nitrite.
The acid is poured into the water contained in a large beaker
(i litre) and the aniline then added. About half the aniline
dissolves as sulphate. The liquid is warmed in the water-bath to
27^and the sodium nitrite, dissolved in a small quantity of water,
is slowly added and the whole well stirred. The temperature
is maintained at 27—30° for a quarter of an hour. As soon as
the sodium nitrite is added the liquid turns yellow and rapidly
becomes turbid from the formation of diazoaminobenzene,
• which separates out in yellowish brown crystalline crusts. The
solution is now allowed to stand at the ordinary temperature
for half an hour, when nearly the whole of the diazoarnino-
bcnzene crystallises out. It is filtered, washed with cold water,
pressed well on the filter, and dried on a porous plate or a pad
of filter paper. It forms a brown sandy powder and may be
purified by recrystallisation from benzene or alcohol. In
crystallising, it is necessary to bring the substance into solution
as quickly as possible. Boiling spirit (about three times the